I’ve thought about this topic for quite some time. Why are some diseases feared more than others? How contagious are they really? For which diseases is the fear truly warranted? And as I considered all the data, it seemed the most logical way to present the information is in a table or two.
Herd immunity threshold estimates are based on the disease’s reproduction number (R0), basically how contagious it is. However, herd immunity is not a fixed number; it is affected by population density, vaccine efficacy, behavior, waning immunity, and pathogen evolution. Highly contagious diseases warrant higher coverage (vaccination) rates, i.e., measles and pertussis. Herd immunity is more difficult to sustain with rapidly mutating viruses like influenza and COVID. Contagiousness is affected by the R0, how many people one infected person can infect in a susceptible population, as well as the mode and ease of transmission.
The R0 is affected by numerous factors:
· Sanitation
· Crowding
· Climate
· Immunity
· Medical care
As such, the R0 figure also has a broad range rather than a fixed constant, and the same is true of the immunity threshold. But what does that R0 actually signify? What numbers are relatively benign, and which suggest our hair is on fire?
|
R0 Interpretation |
|
|
<1 |
Usually dies out quickly |
|
1-2 |
Slow spread |
|
2-5 |
Sustained epidemic potential |
|
5-10 |
Highly contagious |
|
10+ |
Exceptionally contagious |
The modern mortality rate assumes the advantages of modern medicine—good nutrition, advanced hospital care, medications, sanitation. As these become less available, the mortality rate will edge up toward the historical numbers. The herd immunity threshold is the percentage of the population that needs to be immune through natural exposure or vaccination to prevent the spread of the disease.
|
R0, Herd Immunity Thresholds, and Mortality Rates for Major Communicable Diseases |
|||||
|
Disease |
R0 |
Herd immunity threshold, % |
Mortality rate, % |
Notes |
|
|
Historical |
Modern |
||||
|
Measles |
12-18 |
92-95 |
5-15 |
0.1-0.3 |
Extremely contagious, common childhood killer. Deadlier with malnutrition, vitamin A deficiency. |
|
Pertussis |
12-17 |
92-94 |
5-15 in infants |
<1 |
Extremely contagious, deadly to infants |
|
COVID |
8-15 |
60-80 |
-- |
0.5-1 |
Rapid variant evolution; mortality rate age-dependent. |
|
Chickenpox |
8-10 |
85-90 |
0.01-0.05 |
Rare |
Extremely common childhood infection prior to vaccination. |
|
Polio |
5-7 |
80-86 |
2-5 in children; 15-30 in adults |
|
Paralysis and disability, esp. in children. Most infections are mild, asymptomatic. |
|
Rubella |
5-7 |
83-85 |
Low |
Rare |
Major concern is congenital rubella syndrome, which causes fetal mortality and severe birth defects. |
|
Diphtheria |
5-7 |
83-86 |
5-20 |
5-20 |
Outcome depends on quick access to antitoxin, antibiotics, and vaccination status. |
|
Mumps |
4-7 |
75-86 |
0.03 |
Rare |
Rare, usually associated with severe neurological complications or compromised immune system. |
|
Smallpox |
3-6 |
67-83 |
30; hemorrhagic forms 90 |
-- |
R0 up to 10 in dense populations; survivors severely disfigured. |
|
SARS |
2-4 |
-- |
-- |
10 |
High fatality and hospital outbreaks. |
|
Hepatitis B |
2-4 |
80 |
|
|
Death due to cirrhosis or liver cancer takes years to decades. Transmission is through blood, sexual contact, or at birth. |
|
Ebola |
1.5-2.5 |
-- |
-- |
25-90 |
High mortality rate and hemorrhagic symptoms. |
|
Cholera |
1-4 |
-- |
|
50 untreated; 1 treated |
Massive outbreaks due to contaminated water. Death from dehydration. |
|
Tuberculosis |
1-4 |
0-75 |
50, untreated |
Uncommon with access to medications |
R0 is higher in crowded conditions with poor ventilation. Takes years to progress. |
|
Pneumonic plague |
1-4 |
-- |
90-100 |
10-15 with prompt antibiotic treatment |
Respiratory transmission. Often rapidly fatal. |
|
Bubonic plague |
1-3 |
-- |
30-60 |
5-15 with prompt antibiotic treatment |
Black Death. Got antibiotics? |
|
1918 influenza |
1.8-3 |
-- |
2-10 |
-- |
Pandemic, tens of millions of deaths worldwide; more likely fatal in young adults. |
|
Seasonal influenza |
1.2-1.8 |
50-70 |
Varies depending on strain |
<0.1 |
Varies by strain/year. |
|
MERS |
<1 |
-- |
-- |
34 |
High mortality; poor sustained spread; very severe. |
Patterns to take note of:
· The most contagious diseases are usually not the most lethal. (Measles, pertussis/whooping cough, chickenpox)
· Diseases with a lower R0 kill or incapacitate their hosts rapidly before the hosts can spread them further. They usually require close contact or specific transmission pathways (Pneumonic plague, Ebola)
· Airborne diseases generally have the highest R0 (measles, pertussis, chickenpox)
· The diseases perceived as most historically terrifying were those with high mortality rates and severe outcomes (smallpox).
· Measles spreads far more easily than smallpox.
· Smallpox is spread primarily through close face-to-face respiratory contact, contaminated linens or clothing. It is much less contagious than measles. And people are generally contagious only after symptoms begin, which is contained with isolation of the patient.
· TB is distinct in that it spreads through airborne aerosols. Transmission generally requires prolonged indoor exposure. Most people infected develop latent TB, not the actively contagious disease. And infectiousness varies considerably between patients. While it is airborne, it is not highly contagious in casual contact, unlike measles.
Today, mortality rates vary significantly depending on
· Nutrition
· Vaccination status
· Antibiotics, antivirals, etc.
· Supportive care
This, in addition to proper hygiene and sanitation, is where our preparations make a difference.
References:
https://journals.sagepub.com/doi/10.1177/0890117120930536d
